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Appendix II.                                 829

with safety, and having rapid steam-raising properties, created the
Thornycroft and Yarrow boilers, at least for the smaller boats;
after which the Belleville boiler (originally introduced for French
boats in 1879) was naturalised in England for the larger ships.
Meanwhile, the simultaneous development of the cylindrical or
' Scotch' marine boiler proceeded, and now it is a question which
is to hold permanent ground. The consensus of opinion seems
to be in favour of retaining the Scotch boiler for passenger and
cargo service, but especially for the latter, while the special
advantages of the water-tube boiler fit it for war vessels. On
land, the high efficiency of the Lancashire boiler and its steady
steam-supplying properties make it a difficult rival: neither is it
necessary to save space or weight on land. Taking now the
water-tube boiler alone, we may class its advantages :

ADVANTAGES OF WATER-TUBE BOILERS.

1.  Will give higher pressure steam than cylindrical boilers.

2.  Repairs, though probably more frequent, are easily made.

3.  Steam can be raised more quickly from cold water, there

being little risk of deformation.

4.   Circulation is systematic, rising by the tubes, and returning

by 'downcomer/ Circulation, however, does'not in-
crease rate of evaporation, but the reverse: its function
is to keep steam moving and avoid dry plates.

5.  Lighter than other boilers for same power, because heating

surface is largely containing surface as well, whereas a
separate (and heavy) envelope is required for the latter
purpose in other boilers. Also heating surface is lighter
per square foot than in rnany other boilers.

6.  Less space for same power.

7.  Safer at high pressures.    This, however, is dependent on

good circulation and on a fairly small steam reservoir.

8.   Portability: can be carried about in sections and fitted up

afterwards.

The disadvantages of this boiler seem to be its doubtful
economy in regular use, and the fact that only smokeless coal
can be burned. Its greatest advantage is its favouring high